r? @brson

Fix syntax highlighting of code fences

The documentation for RESULT_EXPECT_USED includes this code:

    let res: Result<usize, ()> = Ok(1);
    res?;
    # Ok::<(), ()>(())

Because the code fence didn't start with `rust`, the code wasn't highlighted and the line starting with `#` was displayed on the website. This is now fixed.

EDIT: I noticed that highlighting for some other lints is broken as well. It only works if the code fence looks like this:

````markdown
```rust
// ..
```
````

However, many code blocks were ignored. I un-ignored most code blocks and made them compile by adding hidden code with `#`. While doing so, I found two mistakes:

```rust
opt.map_or(None, |a| a + 1)
// instead of
opt.map_or(None, |a| Some(a + 1))
```
and

```rust
fn as_str(self) -> &str
// instead of
fn as_str(self) -> &'static str
```

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